XII.] THE FRESH- WATER CRAYFISH. 125 



pair consists of the long feelers or antennae : the next, of the 

 short feelers or antennules ; and the most anterior is formed 

 by the short subcylindrical stalks (ophthalmites), on the ends 

 of which the eyes are situated. 



This enumeration shews that the Lobster and Crayfish 

 have six pairs of abdominal appendages the swimmerets ; 

 eight pairs of thoracic appendages (four pairs of ambulatory 

 limbs, one pair of chelae, three pairs of maxillipeds), and six 

 pairs of cephalic appendages (two pairs of maxillaa, one pair 

 of mandibles, one pair of antenna, one pair of antennules, 

 one pair of eyestalks), making in all twenty pairs of append- 

 ages. In correspondence with the number of appendages 

 the body consists of twenty somites ; of which six remain 

 moveable upon one another to form the abdomen, while the 

 other fourteen are united to form the cephalothorax. 



The branchiostegite is an outgrowth of the dorsolateral 

 region of the confluent thoracic somites. The serrated rostrum 

 which ends the carapace is a fixed median prolongation of 

 the dorsal wall of the anterior cephalic somites ; while the 

 telson is a moveable median prolongation of the dorsal wall 

 of the sixth abdominal somite. The labrum and the meta- 

 stoma are median growths of the sterna of the prseoral and 

 post-oral somites. 



Thus the whole skeleton in these animals may be con- 

 sidered as a twentyfold repetition of the ring-like somite 

 with its pair of appendages, which is seen in its simplest form 

 in one of the abdominal somites. Moreover, notwithstanding 

 the great variety of functions allotted to the various ap- 

 pendages, the study of the details of their structure (see 

 Laboratory work) will shew that they are all reducible to 

 modifications of a fundamental form, consisting of a basal 

 joint (protopodite) with three terminal divisions (endopodite, 

 exopodite, epipodite). 



As has been already said, the Lobster and Crayfish are 



